1. Field of the Invention
The present invention relates to a piezoelectric/electrostrictive device, more particularly to a piezoelectric/electrostrictive device for use as an actuator utilizing a flexural displacement, or various types of sensors (e.g., a sensor for a microphone, a viscosity sensor, etc.) for detecting fluid properties, sound pressure, micro weight, acceleration and the like.
2. Description of the Related Art
A piezoelectric/electrostrictive device is used as an actuator or various types of sensors. As such piezoelectric/electrostrictive device, there is disclosed, for example, a device for use in measuring fluid properties such as density, concentration, and viscosity (see Japanese Patent Application Laid-Open No. 8-201265). Such piezoelectric/electrostrictive device is used as a sensor by utilizing correlation between amplitude of the piezoelectric/electrostrictive device as a vibrator and viscous resistance of the fluid brought into contact with the piezoelectric/electrostrictive device (vibrator).
In general, it is possible to replace a vibration configuration such as the vibration of the vibrator in a mechanical system with an equivalent circuit in an electrical system. When the piezoelectric/electrostrictive device (vibrator) is vibrated in a fluid, this vibrator receives a mechanical resistance based on the viscous resistance of the fluid, and accordingly an electric constant of the equivalent circuit of a piezoelectric/electrostrictive element constituting the vibrator changes. By utilizing this, fluid properties such as the viscosity, the density, and the concentration are measured. Here, examples of a measurable fluid include a liquid and a gas. Such liquid may comprise a single component such as water, alcohol, or oil. Examples of the liquid include a liquid obtained by dissolving, mixing, or suspending a soluble or insoluble medium in the component, a slurry, and a paste.
Moreover, examples of the electric constant to be detected include loss coefficient, phase, resistance, reactance, conductance, susceptance, inductance, and capacitance. The loss coefficient or the phase is especially preferably used which has one maximum or minimum change point in the vicinity of a resonance frequency of the equivalent circuit. Accordingly, it is possible to measure not only the viscosity of the fluid but also the density or the concentration thereof (e.g., sulfuric acid concentration in an aqueous sulfuric acid solution). It is to be noted that in addition to the electric constant, a change of the resonance frequency may be utilized as an index for detecting the change of the vibration configuration as long as there is not any special problem from viewpoints of measurement precision and durability. In a case where a ferroelectric is used as the piezoelectric/electrostrictive element, there is detected as a voltage an electric charge generated from a delay of a dipole moment of the ferroelectric after a pulsed electric field is given. A change of the electric charge may be utilized.
As such piezoelectric/electrostrictive device, there is disclosed a device comprising: a ceramic substrate having a thick portion, and a thin diaphragm portion which is formed integrally with the thick portion to form a cavity; and a piezoelectric/electrostrictive element fixed to an outer surface of the substrate. An auxiliary electrode is formed in a position independent of a lower electrode constituting the piezoelectric/electrostrictive element in such a manner that a part of the auxiliary electrode is disposed under a part of the piezoelectric/electrostrictive film (see Japanese Patent Application Laid-Open No. 2002-261347). According to such constitution, an upper electrode can be formed on the surfaces of the auxiliary electrode and the piezoelectric/electrostrictive element continuously without breaking of wire, and reliability of connection of the upper electrode can be enhanced. It is to be noted that a fluid to be measured is introduced into the cavity via a through hole to fill in the cavity. Furthermore, when the auxiliary electrode is continuously formed on not only the outer surface of the thin diaphragm portion but also the thick portion, it is possible to obtain the piezoelectric/electrostrictive device which has stabilized device properties and which is not easily limited by use conditions.
In the piezoelectric/electrostrictive devices disclosed in the above-described patent documents and the like, although the thin diaphragm portion only is to be vibrated in conjunction with driving of the piezoelectric/electrostrictive element, in actual, not only the thin diaphragm portion but also the thick portion are vibrated, vibration energy of the thin diaphragm portion damps, and displacement (amplitude) is decreased. Moreover, since response is lowered, there is a problem that it is difficult to perform high-precision (high-resolution, high-sensitivity) detection.